A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic apparatus, a mask is placed on a mask table, or reticle stage RS, and moved by a positioner under the control of a motion control system. Similarly, a substrate is placed on a substrate table, or wafer stage WS, and moved by a positioner under the control of the motion control system. The movements of the reticle stage RS and the wafer stage WS are coordinated in order to irradiate the desired target portion of the substrate with the desired pattern from the mask. In general, the projection system has a magnification factor M (which generally is <1), and therefore the speed at which the wafer stage WS is scanned will be a factor M times the speed at which the reticle stage RS is scanned. Since wafer stage WS errors tend to be larger than reticle stage RS errors due to, inter alia, on-the-fly levelling actions, the remaining position error ews of the wafer stage WS is coupled feedforward to the reticle stage RS to result in a feedthrough factor ewrs/ews (wafer stage to reticle stage position error in relation to wafer stage position error) which may be optimized as disclosed in U.S. Pat. No. 6,727,977, which is incorporated herein by reference. An additional benefit is that reticle stage RS positioning errors only contribute by a factor of M to an imaging error.
Conventionally, to overcome the delay in the feedthrough, a feedthrough may be optimized for frequencies lower than a threshold frequency, in this particular case 300–350 Hz. However, this improvement of the feedthrough factor for lower frequencies is obtained at the cost of a larger high-frequency gain. Thus, higher frequencies are amplified, and may disturb the overall system performance to generate MSD (Moving Standard Deviation) errors resulting in severe loss of imaging performance on the substrate.